Setting CustomerId and ProductId

Hi,
I am newbie. I would like to ask about a user control named as CustomerSendPrivateMessage.ascx

I am registering the CustomerSendPrivateMessage user control on my aspx page by writing:

<%@ Register TagPrefix="mot" TagName="PM" Src="~/Administration/Modules/CustomerSendPrivateMessage.ascx" %>

and using the control in content place holder

<asp:Content ID="Content1" ContentPlaceHolderID="cph1" runat="server">
    <mot:PM runat="server" ID="ctlPM" />
</asp:Content>

On the codebehind side I would like set CustomerId property of the control by writing

ctlPM.CustomerId = 37;

but compiler complains that :

'NopSolutions.NopCommerce.Web.Administration.Modules.CustomerSendPrivateMessage.CustomerId' cannot be assigned to -- it is read only

Is there anybody who knows how to set CustomerId property in this example?
Thank you.
Inanc